1. Field of the Invention
The present invention relates to a cancelling mechanism for a turning signal which automatically returns an operation lever operated in order to indicate the turning direction of a vehicle to a neutral position by the turning of a steering shaft in the direction opposite from that in which the operation lever is turned.
2. Description of the Prior Art
Cancelling mechanisms for turning signals of the above-described type generally include a bracket formed in such a manner that it surrounds a steering shaft, the bracket being pivotally supported on a base, an operation lever mounted on one side of the bracket, and a ratchet provided at the other side of the bracket, the ratchet serving as the cancelling mechanism. An arrow head "B" in FIG. 6 shows each position of the operating lever 4. When the bracket is displaced by the operation lever to a position which represents a left turn shown by a symbol B-I in FIG. 6 or a right turn shown by a symbol B-III IN FIG. 6, the claw piece of the ratchet is positioned within the locus of the projecting portion of a cancelling cam. The cancelling cam presses the claw piece of the ratchet when it is rotated in the direction opposite from that in which the lever was operated, returning the bracket, and hence the operation lever, to the neutral position shown by a symbol B-II in FIG. 6. However, the above-described cancelling mechanism suffers from a problem in that the size thereof is large due to the structure in which the bracket surrounds the steering shaft.
Accordingly, an improved cancelling mechanism has been proposed in the specification of Japanese Utility Model Laid-Open application No. 57-199146. The structure thereof is shown in the prior art FIG. 6. More specifically, in this cancelling mechanism, a bracket 3 made of a plastic is supported at one side of a steering shaft 1 in such a manner as to be pivotal about a shaft 2. The bracket 3 is pivoted by an operation lever 4 mounted on the side of the bracket 3 opposite from the side which is mounted to the steering shaft 1 between a neutral position and a right turn position or a left turn position. An engaging member 5 is mounted on the side of the bracket 3 which is closer to the steering shaft 1 in such a manner as to be movable. The engaging member 5 has a forward end portion 5a which is directed toward the steering shaft 1. Two engaging arms 6 that can be elastically deformed are also mounted on that side of the bracket 3 in such a manner that they face a proximal end portion 5b of the engaging member 5. When the bracket 3 is pivoted from the neutral position to the right turn position or the left turn position, the forward end portion 5a of the engaging member 5 becomes positioned within a locus of rotation of projecting pieces 1a of the steering shaft 1. In this state, when the steering shaft 1 is rotated in the same direction as that in which the bracket is operated, the proximal end portion 5b of the engaging member 5 is pivoted between the engaging arms 6. On the other hand, when the steering shaft 1 is rotated in the opposite direction, the projecting piece 1a abuts against the forward end portion 5a of the engaging member 5, bringing the proximal end portion 5b thereof into engagement with the engaging arm 6 and thereby returning the bracket 3 to the neutral position. At this time, if the operation lever 4 is manually locked by a driver, the engaging member 5 which abuts against the projecting piece 1a pivots while elastically deforming the engaging arm 6, enabling the projecting piece 1a to pass the forward end portion 5a.
However, in the above-described cancelling mechanism, since the two engaging arms 6 are made elastically deformable so as to cause the engaging member 5 to pivot when the operation lever 4 is locked, they are also elastically deformed in a normal cancelling operation, generating internal stress. Further, the engaging arms 6 made of a plastic become softened at high temperatures, increasing the amount of deformation. These factors, together with the change with time, result in the poor durability of the engaging arms 6, and therefore of the cancelling mechanism.